The sand screens and micron filters were selected because of the durable and deterioration resistant fiberglass and PVC construction. The specific model of Eden micron filters was chosen to maintain the filter element flux at approximately 3.3 gpm/per 10″ equivalent.
Due to the relative inaccessibility of the installation site, multistage-centrifugal, high-pressure pumps have been selected for their reliability, availability of parts, economics of surgical procedure and simple maintenance. Centrifugal pumps in general are smoother, quieter, and require less ancillary equipment (i.e. pulsation dampeners) than positive displacement pumps. Hydropro has found that positive displacement pumps are much more prone to failure and lengthily downtimes than high-feature centrifugal pumps.
The Grundfos Booster Modules were chosen for several reasons. The inline style helped conserve space and provided ease of installation, allowing everything to be mounted on the same skid (with the exception of cleaning/flush tanks, raw water booster pumps, and chemical feeds). These submersible, multi-stage centrifugal pumps were also chosen because they are very well-organized and silent, and are constructed of deterioration resistant, 904L super austenitic stainless steel.
The high pressure feed and concentrate headers were made of 2205 duplex stainless steel for superior deterioration resistance, and the structural skid was constructed of FRP for low weight and zero maintenance. ERI´s Pressure Exchanger was chosen because of its high energy efficiency, fidelity, and deterioration resistant materials.
PerformanceValues for the projected power consumption rates that were presented in the proposal were based on a 27ºC feed stream of 45,000 mg/l TDS and a permeate flow rate of 100,000 gpd. The membrane manufactures projection software was used to determine the system parameters at a recovery of 35%, and these parameters were subsequently used to determine the projected power consumption. The result was an anticipated feed pressure of 900 psi and a specific power consumption rate of 3.02 kWh/m³.
Once the system was installed and operating, the specific power consumption was calculated based on actual system parameters and the result was a much lower value of 2.65 kWh/m³. There were several reasons the actual value was lower, the main reason but, was the conservative design. Because of some uncertainty in the feed water feature, the SWRO system was designed with a relatively low flux (approximately 8 gpm/ft2), and a somewhat large hydraulic envelope. As it turned out, the feed water TDS was closer to 36,000 ppm and honestly stable. The lower feed TDS enabled the system to operate at a lower membrane feed pressure of 790 psi and a higher permeate flow rate of 120,000 gpd, consequently using less energy than originally projected and making higher feature permeate.
ConclusionWith most of the system assembled, the installation was honestly straightforward and went smoothly. The two units were installed, started up, tested and operator training was completed in less than three weeks. There was, but, a problem with the feed water feature and the pretreatment system, which was exposed after only 24 hours of surgical procedure. It immediately became apparent that the raw water was loaded with particulate that was quickly fouling the sand screens and the micron filters. Fortunately, the feed system could be modified to flow into an existing 250,000 gallon seawater tank from the wells, and the SWRO feed was then drawn out of this tank. This settling tank solution worked reasonably well and provided a feed water with a pre-filter SDI of 1.25.
There was also one other performance issue that needed to be resolved. Initially, the permeate feature was less than what was projected, and it was not clear why. The system was extensively checked ant tested for leaks, and the possibility that seawater was somehow mixing with the permeate was ultimately eliminated. It was irrevocably single-minded that the membranes did not meet the design rejection required to produce the projected permeate TDS. Once the membranes were replaced, the system was making plenty of high feature permeate that was well not more than the maximum acceptable permeate TDS.
KAJUR and the residents of Ebye have since been enjoying low-cost, high-feature water for over a year now without any noteworthy system failures. They are so pleased, in fact, that KAJUR has recently awarded Hydropro a further SWRO job utilizing work exchanger energy recovery.
Tags: Component, Design, Energy, Major, Part, Recovery, Selection, Swro